Nonlinear wave growth theory of whistler-mode chorus and hiss emissions in the magnetosphere

Nonlinear processes associated with the generation process of whistler-mode chorus emissions are summarized. The nonlinear dynamics of energetic electrons interacting with a coherent whistler-mode wave and the formation of electromagnetic electron holes or hills in the velocity phase space are described. The condition for resonant electrons to be free from the anomalous trapping at low pitch angles is obtained. In the presence of the inhomogeneity due to the frequency variation and the gradient of the magnetic field, the electron holes or hills result in resonant currents generating rising-tone emissions or falling-tone emissions, respectively. After formation of a coherent wave at a frequency of the maximum linear growth rate, triggering of the nonlinear wave growth takes place when the wave amplitude is above the threshold amplitude. The wave grows to a level close to the optimum wave amplitude as an absolute instability near the magnetic equator. The nonlinear growth rate at a position away from the equator is derived for a subtracted Maxwellian momentum distribution function with correction to the formulas in the past publications. The triggering process is repeated sequentially at progressively higher frequencies in the case of a rising-tone emission, generating subpackets forming a chorus element. With a higher plasma density as in the plasmasphere, the triggering of subpackets takes place concurrently over a wide range of frequency forming discrete hiss elements with varying frequencies. The mechanism of nonlinear wave damping due to quasi-parallel propagation from the equator is presented, which results in the formation of a gap at half the electron cyclotron frequency, separating a long rising-tone chorus emission into the upper-band and lower-band chorus emissions. The theoretical formulation of an oblique whistler mode wave and its interaction with energetic electrons at the n-th resonance is also presented along with derivation of the inhomogeneity factor.

The nature of the variability of wave particle interactions in the inner magnetosphere and consequences for diffusion models

<p>It is important to understand the variability of plasma processes across many different timescales in order to successfully model plasma in the inner magnetosphere. In this presentation, we focus on the interplay between the variability cold plasmaspheric plasma, whistler-mode wave activity, and the efficacy of wave-particle interactions in the inner magnetosphere. We use in-situ observations to quantify the amount and timescales of variability in pitch-angle diffusion due to plasmaspheric hiss in Earth’s inner magnetosphere, and suggest reasons for the variability. We then use a stochastic parameterization scheme to investigate the consequences of that variability in a numerical diffusion model. The results from the stochastic parameterization are contrasted with the standard approach of constructing averaged diffusion coefficients. We demonstrate that even when the average diffusion rates are the same, different timescales of variability in the wave-particle interactions lead to different end results in numerical diffusion models. We discuss the implications of our results for the modelling of wave-particle interactions in magnetospheres, and suggest quantifications that are vital for accurate modelling.</p>

Energetic electron precipitation via oblique whistler mode chorus emissions in the outer radiation belt

<p>Whistler mode chorus emissions in the Earth’s magnetosphere cause energetic electron precipitation and the associated pulsating aurora. First-order cyclotron resonance in parallel whistler mode wave-particle interactions is the main mechanism of the precipitation. Not only cyclotron resonance but also Landau resonance and higher-order cyclotron resonances occur in the oblique whistler mode wave-particle interactions. Especially, electrons can be accelerated and scattered to lower equatorial pitch angles rapidly via Landau resonance. We apply test particle simulation and the Green’s function method to check the energetic electron precipitation caused by oblique chorus emissions. We simulate the wave-particle interactions around L=4.5 for electron ranges from 10 keV to a few MeV. We further compare the precipitation fluxes between parallel and oblique chorus emissions. Our simulation result reveals that oblique chorus emissions lead to more electron precipitation than parallel chorus emissions. At kinetic energy E < 100 keV, the electron precipitation ratio (oblique case/parallel case) is about 1.3. At 100 keV < E < 0.5 MeV, the ratio is greater than 2. At E > 0.5 MeV, the ratio is greater than 2 orders. Multiple resonances effect in the oblique whistler mode wave-particle interactions is the reason for the greater precipitation.</p>